I was using the mod Interdimensional on a 1.18.1 Fabric 0.12.12 server, and Immersive Portals 1.1.8 with Polymer 0.2.0-beta.9+1.18.1 via interdimensional
The error was this: https://pastebin.com/QuDz5jm2

Install latest version of polymer. You can get link frow readme

The issue is that the polymer library is embedded in the Interdimensional mod. I have also created an issue on their repository though.

You can still download it and drop into mods folder

Ok, that has worked. Thanks!